The present invention relates to a method for screening substances capable of having a therapeutic action in the treatment of transmissible subacute spongiform encephalopathies (TSSEs) or so-called prion diseases, which comprises a step of isolating the PrPres from the spleen; the present invention also relates to methods for isolating the PrPres, which are particularly suited to the said screening method, and their applications in particular in the detection of PrPres.
Transmissible subacute spongiform encephalopathies are caused by nonconventional transmissible agents (NCTAs), also called prions, whose precise nature remains unknown to date. TSSEs comprise essentially Creutzfeldt-Jakob disease, in humans (CJD), scrapie, in sheep and goats, and bovine spongiform encephalopathy (BSE), in bovines; other encephalopathies have been demonstrated in mink or some wild animals such as red deer and elk.
The progression of these diseases is always fatal and there is currently no effective treatment.
In transmissible subacute spongiform encephalopathies, there is an accumulation of a host protein, PrP (or prion protein), in an abnormal form (PrPres), mainly in the central nervous system; PrPres copurifies with the infectivity and its accumulation precedes the appearance of histological lesions. In vitro, it is toxic for neuron cultures.
Two biochemical properties make it possible to distinguish PrPres from the normal PrP: PrPres is partially resistant to proteases and is insoluble in nonionic detergents such as Triton-X100.
The search for new molecules which may be effective in the treatment of these encephalopathies is hindered both by the absence of efficient in vitro models and the length of time for setting up in vivo experimental models, such as experimental scrapie in hamsters (80 to 365 days), or experimental scrapie in mice (180 to 550 days).
Consequently, the inventors set themselves the objective of providing an effective and reliable method of screening which does not exhibit the disadvantages of the experimental models currently used and which is more suitable for the requirements of practical use, in particular in that the evaluation of the action of the substances to be tested can be carried out in less than two months.
To do this, the inventors have found a reliable marker and have developed a reproducible protocol.
The subject of the present invention is a method for screening substances capable of having therapeutic action in the treatment of transmissible subacute spongiform encephalopathies (TSSEs or so-called prion diseases), characterized in that it comprises the following steps:
a) inoculation at time tA, into at least one laboratory animal such as a rodent, mouse or hamster (preferably several, divided into batches), by any appropriate route, of a nonconventional transmissible agent (NCTA) or prion;
b) administration to the said laboratory animal, by any appropriate route, of either a substance to be screened (test animal), or of a placebo (negative control animal), within a period between tAxe2x88x9215 days and tC, corresponding to the time when the PrPres level in the spleen of the said laboratory animal is at maximum or within a period between tB, corresponding to the time of the first detection of PrPres in the spleen of the said laboratory animal and tC; tB being between tA and tA+15 and tC being between tA+20 and tA+30, preferably between tA+25 and tA+30;
c) sacrificing of the animals within a time interval between tB and tC, preferably at tC, and collecting of the spleen;
d) isolation of the PrPres from the spleens collected, according to a suitable method of isolation comprising the homogenization of the spleen, followed by a specific extraction of the PrPres comprising a single separation step, from the homogenate obtained, and optionally the purification of the PrPres;
e) semiquantification of the PrPres obtained in step (d) by detection of the said PrPres by any appropriate method, producing a specific signal, followed by a comparison of the signal obtained with a calibration series of dilutions of a positive control consisting of a brain homogenate from an animal at the terminal stage of the disease; and
f) selection of the screened substances as a candidate for the treatment of transmissible subacute spongiform encephalopathies, if the PrPres, level obtained in the spleen of the test animal, in step e), is reduced by at least a factor of 2 compared with the level obtained under the same conditions with the negative control animal.
The times tA, tB and tC are expressed in days; tA=D0.
Indeed, the inventors have found, unexpectedly, that substances which increase the survival of infected animals, regardless of the mode of inoculation (peripheral or intracerebral), also result in a delay in the accumulation of PrPres in the spleen, detected under standardized conditions.
Standardized conditions are understood to mean, for the purposes of the present invention, conditions in which the following parameters are selected:
NCTA selected,
route of administration of NCTA,
method of isolation of the PrPres from the spleen.
For a strain selected from a given animal, at the terminal stage of the disease, the infectious titer is constant.
The detection of PrPres in the spleen makes it possible to observe much more rapidly the effects of the molecules to be tested, in particular the inhibition of the accumulation of PrPres, either within the hours following the inoculation (capture of the inoculum by the spleen and detection of a PrPres peak between tA and tA+1-2 days), or between 5 and 15 days after the infection; tB corresponds both to the detection of the peak at the time of capture of the inoculum and to the detection of the neosynthesized PrPres; this is the reason why tB is between tA and tA+15; these tB and tC values may vary within the ranges defined above, depending on the NCTA and the laboratory animal (mouse) selected; for example, when the UCTA corresponds to the murine strain C506M3 inoculated by the intraperitoneal route, in the C57BL/6 mouse, the neosynthesized PrPres may be detected in 100% of cases, from the 5th day post-infection (p.i.) (tB) and a plateau is observed from the 30th day p.i. (tC).
Such a method therefore makes it possible to select molecules capable of preventing the accumulation of PrPres; such molecules are considered to be capable of exhibiting therapeutic action in the treatment of TSSEs.
In accordance with the invention:
In step a):
the NCTA corresponds to a strain stabilized in the host animal, that is to say which exhibits stable characteristics in this host animal after several passages and in particular the following characteristics: identical period before the appearance of the disease and identical lesional profile during passages in all animals (degree of vacuolation of various parts of the brain); it corresponds to any strain stabilized under the abovementioned conditions, inducing a premature accumulation of PrPres in the spleen of the host animal, such as scrapie strains or bovine encephalopathy strains, in particular the scrapie strains called Chandler, ME7, 139A (M. E. Bruce et al., Scrapie strain variation and its implication in Current topics in Microbiology and Immunology: Transmissible Spongiform Encephalopathies, Scrapie, BSE and related disorders, 1991, 172, 125-138), C506M3 (C. I. Lasmxc3xa9zas et al., J. Gen. Virol., 1996, 77, 1601-1609) or 263K (R. H. Kimberlin et al., J. Gen. Virol., 1977, 34, 295-304 and 1978, 39, 487-496) or the BSE strains called 4PB1 (C. I. Lasmxc3xa9zas et al., 1996, cited above) and 301V (C. F. Farquhar et al., J. Gen. Virol., 1996, 77, 1941-1946);
the said NCTA is preferably administered in a buffer suited to the route of administration selected in the form either of a crude tissue, preferably brain, homogenate, or of a PrPres pellet, obtained by appropriate centrifugation, from a crude tissue, preferably brain, homogenate;
the said NCTA may be administered by any route (oral route, parenteral route), preferably by the intraperitoneal route, at a dose corresponding to an inoculum of NCTA, between 0.001% and 10% (weight/volume) (LD50 between 103 and 107);
the said laboratory animal is preferably a rodent (mouse or hamster, for example).
In step b):
the substance to be screened is administered by the oral or parenteral route;
if the treatment is started between tB and tC, (that is to say when the PrPres in the spleen is constantly detectable), the model according to the invention makes it possible to study only the action of the substance to be screened on the NCTA inoculated during replication at the sites of replication (target cells), whereas if it is administered before tB, for example at tA, the model according to the invention makes it possible to study, in addition, the action of the substance to be screened before the NCTA has reached its target cells in the spleen.
In step d):
depending on the sequence of steps selected among the known protein isolation techniques, namely the methods based on molecular size, such as centrifugation, the methods based on differences in solubility such as salting in and salting out or fractionation by solvents or the methods based on electrical charge, the degree of purification and the yield will be different. Within the framework of the present invention, it is necessary to select a reliable and sensitive method which makes it possible to obtain a detection threshold such that the ratio: maximum level detectable in the spleen/cut off is as high as possible, preferably greater than 2 or such that when a xc2xd dilution of the final sample obtained is carried out, a detection signal is still obtained;
sequences of steps preferred are described hereinafter: they have the advantage, over the methods of isolation previously described, of having high reliability and high sensitivity, because the actual extraction comprises only a single separation step and because of the particular selection of the sequence of steps, whereas in the methods previously described (R. E. Race et al., J. Gen. Virol., 1992, 73, 3319-3323; Doi et al., J. Gen. Virol., 1988, 69, 955-960; T. Muramoto et al,. Am. J. Pathol., 1993, 143, 5 1470-1479; Farquhar C. F. et al., Gen. Virol., 1994, 75, 495-504 and J. Gen. Virol., 1996, 77, 1941-1946), the extraction comprises several separation steps and leads to an imprecision as regards the quantification, and/or the sensitivity of these methods is insufficient to obtain a fine detection threshold and a fine quantification and in particular to effectively detect a large variation in the PrPres level.
In step e):
the PrPres is in particular detected by immunoassay (Western blot for example).
The subject of the present invention is also a method for isolating PrPres, from an organ or a tissue, in particular the spleen or the brain, characterized in that it comprises essentially the following steps:
(i) homogenization of organ or tissue, collected after sacrificing the animal, by mechanical grinding in a homogenization buffer, followed by calibration of the homogenate, for the production of a homogenate comprising, in weight/volume, from 5 to 50% of the said organ or tissue;
(ii) specific extraction of PrPres comprising a single separation step, by treating the homogenate obtained in step (i) by incubating the suspension obtained with a protease and an anionic detergent (surfactant), capable of promoting the aggregation of the PrPres, such as 10-30% sarkosyl (lauroyl sarcosine) in a suitable buffer and separation of the PrPres, by a single ultracentrifugation at 480,000-1,200,000 g.h, preferably for 2-4 hours, for example at 240,000-300,000 g for 2 to 4 h, preferably at 20-22xc2x0 C., of the suspension obtained, deposited on a buffer cushion having a density of between 1.02 and 1.08, at 20xc2x0 C. and recovering the centrifugation pellet comprising the said PrPres; and, if necessary,
(iii) purification of the PrPres by suspending the centrifugation pellet obtained in (ii) in a Laemmli buffer comprising 1-5% SDS, incubating in this buffer at 100xc2x0 C. for 2-10 minutes and centrifuging at 12,000-15,000 g for 10-15 minutes at 16-22xc2x0 C.
The said PrPres thus purified may then be separated by any appropriate technique such as electrophoresis (polyacrylamide gel electrophoresis, for example) or immunocapture, from the centrifugation supernatant.
In accordance with this method, the homogenization buffer in step (i) is in particular a neutral buffer such as water or an isotonic buffer such as 5% glucose.
Also in accordance with the invention, during the extraction step (ii), the ultracentrifugation is carried out after depositing the suspension containing the PrPres on a 6-20% sucrose cushion.
As a variant, the subject of the present invention is also a method in which the extraction comprises a single step for the preparation of the PrPres, and does not require ultracentrifugation; such a method for isolating PrPres, from an organ or tissue, in particular the spleen or the brain, is characterized in that it comprises essentially the following steps:
(i) homogenization of organ or tissue, collected after sacrificing the animal, by mechanical grinding in a homogenization buffer, followed by the addition, to the homogenate obtained, of a salt having a high ionic strength and capable of promoting the aggregation of the PrPres, such as 10-30% NaCl, in a 1:1 (v/v) ratio, followed by calibration of the homogenate, for the production of a homogenate comprising, in weight/volume, from 5 to 50% of the said organ or tissue;
(ii) specific extraction of PrPres by treating the homogenate obtained in step (i) by incubating the suspension obtained with a protease and an anionic detergent capable of promoting the aggregation of the PrPres, such as 10-30% sarkosyl and a single separation of the PrPres, by centrifugation at 25,000-60,000 g.h, for example at 25,000-30,000 g for 1 to 2 h, preferably at 16-22xc2x0 C., of the suspension obtained, deposited on a buffer cushion having a density of between 1.02 and 1.08, at 20xc2x0 C., and recovering the centrifugation pellet comprising the said PrPres; and, if necessary,
(iii) purification of the PrPres by suspending the centrifugation pellet obtained in (ii) in a Laemmli buffer comprising 1-5% SDS, incubating in this buffer at 100xc2x0 C. for 2-10 minutes and centrifuging at 12,000-15,000 g for 10-15 minutes at 16-22xc2x0 C.
The said PrPres thus purified may then be separated by any appropriate technique such as electrophoresis (polyacrylamide gel electrophoresis, for example) or immunocapture, from the centrifugation supernatant.
In accordance with this method, the homogenization buffer in step (i) is in particular a neutral buffer such as water or an isotonic buffer such as 5% glucose.
Also in accordance with the invention:
during the extraction step (ii), the solution used for the extraction comprises an anionic detergent capable of promoting the aggregation of the PrPres and a detergent having protein-renaturing properties, such as a zwitterionic detergent, such, as a sulphobetaine, preferably the sulphobetaine SB 3-14 at 1-2%, in a 1:1 (v/v) ratio;
during the extraction step (ii), but prior to the centrifugation, at least one protease inhibitor is added;
the centrifugation depending on the extraction step (ii) is preferably carried out after depositing the suspension containing the PrPres on a 6-20% sucrose cushion or a 6-20% sucrose cushion and a sulphobetaine.
The PrPres can then be detected by any appropriate specific method.
Surprisingly, these methods for isolating PrPres, from the spleen, comprising an extraction in a single step, do not bring about a cumulative loss of PrPres and can be directly used without modification, to extract the PrPres from any other tissue.